I run a single 3" cat with my set up. It has a very noticeable drop in power with the cat on it. Turbo is slower to spool due to the increase in backpressure. I have not had any issues so far with just the single cat and emissions testing. I have OBDII testing for my 96 and have yet to be hassled for the visual test.
96 GT Vert, 76mm turbonetics, built 4r70w w/brake, UCC Converter, 60lb inj., ba2400, 10.96 @ 124 w/19psi, stock heads, stock intake, stock exhaust manifolds, stock tb. Tuned with a Tweecer